Moire is an artifact that results whenever two geometrically-regular patterns are superimposed. This artifact often manifests itself as a ripple-like pattern in scanned representations of screened images because the geometrically-regular structure of the screened image is superimposed with the geometrically-regular pattern of scanning.
The nature of the moire that results from scanning depends on a number of factors including: (1) the resolution of the geometrically-regular structure of the screened image, referred to herein as the screen frequency, (2) the resolution of the geometrically-regular pattern for the scanning process, referred to herein as the scan frequency, (3) the relative angle between these two patterns, referred to herein as the screen angle, and (4) the characteristics of the process used to render the scanned representation. Moire can be reduced by decreasing the screen frequency, increasing the scan frequency, and/or carefully selecting the screen angle, but these simple techniques are generally unsatisfactory.
A tradeoff between screen frequency and scan frequency is generally unable to achieve a satisfactory reduction in moire because, on the one hand, a decrease in screen frequency reduces the resolution of the image and, on the other hand, an increase in scan frequency greatly increases the amount of resources required to process and store the scanned representation. In some applications such as high-quality printed publications, screen frequencies in excess of 200 lines per inch (about 79 lines per centimeter) are used, making it essentially impossible to reduce moire by merely increasing the scan frequency.
A careful selection of screen angle is generally unable to achieve a satisfactory reduction in moire because the angle is difficult to select precisely in many applications. Precise selection is difficult because the relative orientation of the screened image with respect to the orientation of the scanning apparatus is difficult to control precisely. The problem is even greater in color applications where the screen angle for three or more colors must be very carefully selected.
A variety of techniques exist that can reduce moire by reducing the regularity of the pattern in either or both the image screen and the scan. These techniques usually are not satisfactory because it is often impossible to control how the screen image is produced and irregular scanning patterns are difficult to implement.
A variety of other techniques attempt to reduce moire by using so called stochastic screens to introduce a degree of randomness in either a scanned representation or a printed replica of a scanned representation. These techniques are not attractive because considerable processing resources are required to perform the quasi-stochastic processes needed to implement the stochastic screens. In addition, a considerable amount of random-access memory (RAM) is required for these processes to operate on the scanned representations.
A variety of other techniques reduce moire by applying a low-pass filter to the scanned representation. If a filter is applied uniformly to the representation, however, the degree of filtering required to achieve an acceptable reduction in moire usually results in an unacceptable reduction in resolution. For this reason, a number of adaptive filtering techniques have been developed. According to one technique, different filters are applied to a scanned representation and weighted combinations of the filter outputs are formed according to whether edges or other high-spatial-frequency components are present in the image. This techniques are sometimes well suited for certain specialized applications like enhancing X-ray images for examination by trained medical personnel but are not as well suited for high-quality reproduction of arbitrary images.
According to another technique, the scanned representation is filtered by a spatially-variant filter that is adapted according to the presence and orientation of edges in the image. This technique is not attractive because considerable processing resources are required to implement the spatially-variant filters. In addition, considerable memory is required for these filters to operate on the scanned representations.
All known techniques for reducing moire are deficient because they cannot achieve high-quality reproduction of all types of screened images using essentially any screen and scan frequency and/or screen angle for display by essentially any display technique. Furthermore, as mentioned above, implementation of many of these techniques require an amount of processing and memory resources that cannot be provided inexpensively. These known techniques also do not work well for images that include periodic structures of multiple screen frequencies.